Catalytic hydroprocessing refers to petroleum refining processes in which a carbonaceous feedstock is brought into contact with hydrogen and a catalyst, at a higher temperature and pressure, for the purpose of removing undesirable impurities and/or converting the feedstock to an improved product.
Heavy hydrocarbon feedstocks can be liquid, semi-solid and/or solid at atmospheric conditions. Such heavy hydrocarbonaceous feedstocks can have an initial ASTM D86-12 boiling point of 600° F. (316° C.) or greater.
The feedstock properties that influence its hydroprocessability include: organic nitrogen content, especially basic nitrogen content; feed boiling range and end point; polycyclic aromatics content and previous processing history (i.e., straight run versus thermally cracked).
Heavy hydrocarbonaceous oils boiling in the gas oil range can be high in heteroatom content, especially nitrogen. Nitrogen content can range from about 50 ppmw to greater than 5000 ppmw elemental nitrogen, based on total weight of the heavy hydrocarbonaceous oils. The nitrogen containing compounds can be present as basic or non-basic nitrogen species. Examples of basic nitrogen species include pyridines, alkyl substituted pyridines, quinolones, alkyl substituted quinolones, acridines, alkyl substituted acridines, phenyl and naphtha substituted acridines. Examples of non-basic nitrogen species include pyrroles, alkyl substituted pyrroles, indoles, alkyl substituted indoles, carbazoles and alkyl substituted carbazoles.
Heavy hydrocarbonaceous oils boiling in the gas oil range can have sulfur contents ranging from about 500 ppmw to about 100,000 ppmw elemental sulfur (based on total weight of the heavy hydrocarbonaceous oils). The sulfur will usually be present as organically bound sulfur. Examples of such sulfur compounds include the class of heterocyclic sulfur compounds including but not limited to thiophenes, tetrahydrothiophenes, benzothiophenes and their higher homologues and analogues. Other orgranically bound sulfur compounds include aliphatic, naphthenic and aromatic mercaptans, sulfides, disulfides and polysulfides.
Gas oil range feeds contain polycyclic condensed hydrocarbons having two or more fused rings. The rings can either be saturated or unsaturated (aromatic). For the latter, these polycyclic condensed hydrocarbons are also called polynuclear aromatics (PNA) or polyaromatic hydrocarbons (PAH). The light PNAs, with two to six rings, are present in virgin vacuum gas oil streams. The heavy PNAs (HPNA) generally contain 7-10 rings, but can contain higher amounts including 11 rings or at least 14 rings or dicoronylene (15-rings) or coronylenovalene (17-rings) or higher.
Hydrocracking is an important refining process used to process manufacture middle distillate products boiling in the 250-700° F. (121-371° C.) range, such as, kerosene, and diesel. Hydrocracking feedstocks contain significant amounts of organic sulfur and nitrogen. The sulfur and nitrogen must be removed to meet fuel specifications.
Generally, conventional hydrocracking catalysts are composed of (1) at least one acidic component which can be crystallized aluminosilicate and/or amorphous silica alumina; (2) a binding material such as alumina, titania, silica, etc; and (3) one or more metals selected from Groups 6 and 8-10 of the Periodic Table, particularly nickel, cobalt, molybdenum and tungsten.
There are two broad classes of reactions that occur in the hydrocracking process. The first class of reactions involves hydrotreating, in which impurities such as nitrogen, sulfur, oxygen, and metals are removed from the feedstock. The second class of reactions involves hydrocracking, in which carbon-carbon bonds are cleaved or hydrocracked, in the presence of hydrogen, to yield lower boiling point products.
Hydrocracking catalysts are bifunctional: hydrotreating is facilitated by the hydrogenation function provided by the metal components, and the cracking reaction is facilitated by the solid acid components. Both reactions need the presence of high pressure hydrogen.
During hydrocracking, the heavy hydrocarbon feed molecules form a liquid film and covers the active sites of the catalyst. Due to the limitation of hydrogen solubility in hydrocarbons, the hydrogen availability in the hydrocracking catalyst extrudates has been an issue. In practice, the heavy hydrocarbon feed fills the pores first, and reactant hydrogen must then access the active sites in the pores via diffusion through the heavy hydrocarbon feed. Conventional hydrocracking catalysts exhibit limited hydrogen pore diffusivity with heavy, more refractive feedstocks. This has inhibited the hydrogenation function of the hydrocracking catalysts, which results in middle distillates and unconverted oil (UCO) products with poor quality. This issue becomes more significant when the hydrocracking feed become more disadvantaged, as these feeds consume greater amounts of hydrogen during hydroprocessing, making even less hydrogen available for diffusion into the pores.
Accordingly, there is a current need for a hydrocracking catalyst that exhibits a higher degree of hydrogen efficiency, and greater product yield and quality, as compared to conventional hydrocracking catalysts.